With the new Overwatch stuff showing up on the PTR, we got to see a little more into Mei's backstory with it and it's pretty depressing. We knew she was in stasis for years due to an emergency, and that she is the only survivor of her team. It seems to be that when she awoke, she stayed there for awhile, by herself, her dead team still there, and had to survive till she was eventually rescued, with her bubbly personality being something that likely hides a depression she could struggle with.
As colorful and cartoony this game can be, it's kinda wild how dark the game really is. I think it's made out to be that Winston lost the human who raised and took care of him, due to a revolt of intelligently engineered gorillas. Hanzo nearly killed Genji, regretting it, while Genji was basically turned into a cyborg who I think needs the suit to survive. Something happened with Reaper and Soldier 76 that lead to Reaper nearly dying and left in whatever state he is in now, which doesn't sound good, and Jack was badly wounded and forced to fake his death as Overwatch collapsed. Ana lost her eye and basically went into hiding also, faking her death. Widowmaker was brainwashed, altered, and made to kill her husband where she now works as a mercenary. Bastion is a former killing machine. Tracer was an eager test pilot for a new technology, but it malfunctioned and essentially was beginning to phase out of reality or the time, and can only stay whole if she has that blue chest piece on that was made by Winston. Junkrat and Roadhog are from a literal Mad Max-like wasteland due to the destruction of a fusion plant in Australia who are murderous robbers/mercenaries. Zarya grew up in surrounded by post-war destruction as a child and was forced to abandon her weightlifting/bodybuilding career to enlist as a soldier and protect her home when the attacks began again. Reinhardt has been a part of two groups that were destroyed, the Crusaders seen left strewn around the castle, and saw the collapse of Overwatch and thought that he lost many of those he considered his friends.
It's great to see the world slowly grow for Overwatch, because there's so much hiding there that's only really known through bios on the website, but with the comics and shorts, we get a little bit more all the time.